The present invention relates to a new and improved construction of electrical gas-blast switch.
The gas-blast switch of the invention is the type comprising a movable, substantially tubular-shaped contact element which can be brought into and out of engagement with a fixed set of contacts which, in turn, possesses a rim of rated current contacts. In the cut-on position of the switch the rated current contacts engage about the movable contact element. Further, there is provided a burn-off rim electrically connected with the rated current contacts as well as an intermediate electrode surrounded by the rim of rated current contacts, however electrically insulated therefrom.
With a state-of-the-art switch of this type, as disclosed in Swiss Pat. No. 574,673, the burn-off rim, constructed as a ring, is arranged ahead of the rated current contacts. This burn-off rim is electrically connected with the rated current contacts by means of a coil coaxially arranged with respect to the lengthwise axis of the switch or by means of an electrically conductive contact flange which is in direct contact with the rated current contacts. The intermediate electrode, arranged to be electrically insulated from the rated current contacts, extends past the rated current contacts and the burn-off ring. During the course of the cut-off movement there occur, with the prior art switch, the following operations: Initially, the movable contact element is brought out of engagement with the rated current contacts, but however remains practically in engagement with the burn-off ring, so that the cut-off current flows through the coil arranged forwardly of the burn-off ring. This coil, in turn, produces a magnetic field which is essentially coaxial with respect to the switch axis. If the movable contact departs from the burn-off ring, then initially there is drawn an arc between the movable contact element and the burn-off ring. The contact side-base point of the arc is driven into the interior of the movable contact element owing to the pressure surge which is produced by the arc itself. With increased spacing between the movable contact element and the burn-off ring the arc commutates to the intermediate electrode, thereby forming a two-part arc, namely, a first arc part or portion between the burn-off ring and the intermediate electrode and a second arc part or portion between such intermediate electrode and the movable contact element. Now since every arc strives to move along the path of least resistance, with the heretofore known gas-blast switch, owing to the coil (impedance) connected forwardly of the burn-off ring, there is not insured in any way that the one base point of the aforementioned first arc part will remain at the burn-off ring and will not commutate back to the rated current contacts which--since they are no longer forced apart by the movable contact element--have a smaller radial spacing with respect to the intermediate electrode than the inner diameter of the burn-off ring. What results is an undesired burn-off of the contact surfaces of the rated current contacts.